Joining of aluminum by brazing is a well known process due to the strong and uniform joints that can be produced between aluminum parts of varying shapes and types. There are four major brazing processes utilized for the joining of aluminum parts, these are: (a) the flux dip brazing process wherein the parts to be joined are dipped into a molten flux bath utilizing a mixture of chloride and fluoride salts; (b) the furnace brazing process which employs a small amount of flux, for example a chloride salt; (c) the controlled atmosphere brazing process which uses a small amount of fluoridic salt and an inert gas atmosphere, for example nitrogen, argon or helium; and (d) the vacuum brazing method which uses no flux but instead utilizes a reduced pressure atmosphere for the joining of the aluminum parts.
Each of these brazing methods has advantages and disadvantages. For example, the flux dip brazing process is associated with environmental problems arising out of the disposal of the used flux baths. Also, the aluminum parts joined by the flux dip brazing process must be thoroughly cleaned after fluxing to avoid the corrosive effects of the residual flux on the aluminum surfaces.
In the furnace brazing process, much less flux is utilized and the flux is directly deposited on the surfaces of the parts to be joined. Thus, there is no flux bath disposal problem. The flux remaining on the surface must be removed from the brazed surfaces to minimize the corrosive effects of the flux. Nevertheless, the furnace brazing process cannot be readily utilized for the brazing of those aluminum alloys which have a relatively high magnesium content. Typical examples of those alloys which are not readily brazeable by the furnace brazing method are those aluminum alloys which belong to the Aluminum Association 5XXX series.
The controlled atmosphere brazing process employs an inert gas atmosphere, for example argon or nitrogen gas atmosphere, in the brazing furnace. The inert gas atmosphere brazing employs a relatively small quantity of non-corrosive flux which need not to be cleaned from the brazed surfaces. The fluoridic flux is expensive and in composite brazing sheets undesirable interactions between the fluoride flux and magnesium limit the maximum core alloy magnesium content to about 0.3%.
In vacuum brazing no flux is employed and the method is suitable for joining those aluminum alloys which contain about 0.1-1.75% by weight magnesium or even more. Due to the magnesium content of the aluminum alloy core, the brazed assembly is capable of exhibiting higher strength properties. Vacuum brazing requires a well sealed furnace, careful control of the pressure within the furnace, both of which may impart higher costs to the brazing process. Additionally, in the vacuum brazing process, assembly tolerances must be critically controlled and the cleanliness of the parts is imperative.
For many applications, especially where strength was a major consideration, the use of aluminum alloys containing magnesium (Mg) up to about 2.00% was desired. Joining of such magnesium-containing alloys by brazing could only be accomplished through use of the vacuum brazing process. Vacuum brazing however, requires the installation of an expensive vacuum brazing furnace and thus the process becomes capital intensive.
Those aluminum alloys which are essentially Mg-free cannot be brazed by the vacuum brazing process. Currently, for joining these Mg-free aluminum parts the controlled atmosphere brazing method, employing for example nitrogen atmosphere, is used in the presence of a fluoridic flux. Where brazing of both Mg-free and Mg-containing aluminum alloys was practiced, it was necessary to segregate the different types of alloys and additionally, two different types of furnaces had to be installed, one for controlled atmosphere brazing and the other for vacuum brazing.
Thus, there has been a longstanding need for a filler alloy which could be utilized for the brazing of either magnesium-free or magnesium-containing aluminum alloy parts by controlled atmosphere brazing or by vacuum brazing. Surprisingly, it has been found that an aluminum filler alloy, containing a controlled quantity of lithium can be readily employed for the brazing of Mg-free and Mg-containing aluminum alloys using either the controlled atmosphere (inert gas) brazing method or the vacuum brazing process. The aluminum filler alloy of the invention contains from about 0.01 to about 0.30 % by weight of lithium and as a major alloying element silicon, generally within the limits from about 4 to about 18% by weight of the brazing alloy.
It has been recommended in U.S. Pat. No. 3,272,624 (Quaas) to incorporate 0.005-0.010% lithium into aluminum in order to obtain a self-fluxing filler alloy for welding aluminum parts together. The alloy is employed as an extruded or cast wire and is melted during the joining process to obtain a self-fluxing, deoxidizing deposit in the joint area. If desired, up to 18.0% silicon can also be incorporated in the filler alloy. This alloy is employed as a substitute for fluxes containing chloride and fluoride salts since its residue does not need to be removed from the produced joint. Recommended areas of application include carbon arc, oxy-acetylene and inert arc welding. There is no recognition that the presence of the lithium in the aluminum alloy would render it suitable for use as a filler alloy for the brazing of Mg-containing aluminum parts in the presence of fluxes or as a filler alloy in the fluxless vacuum brazing of aluminum components.
U.S. Pat. No. 4,173,302 (Schultze et al) recommends the use of an aluminum brazing alloy which contains 4-20% silicon and between 0.00001 and 1.0% by weight, preferably between 0,005 and 0.1 by weight at least one of the elements of sodium, potassium and lithium. According to this reference the alloy can be utilized in the fluxless brazing of aluminum-containing articles in a non-oxidizing atmosphere or in a low vacuum. The addition of these alkali metals to the brazing alloy is claimed to increase the corrosion-resistance of the brazed joint. The use of these alkali metal-containing brazing alloys is restricted to fluxless, controlled atmosphere brazing and the beneficial effects of these alkali metals are considered equivalent.
U.S. Pat. No. 5,069,980 (Namba et al) describes a clad aluminum alloy suitable for fluxless vacuum brazing. The cladding material is to be used on both sides of a core sheet. It contains 6-14% silicon, 0-0.6% magnesium, balance aluminum and additionally, at least one of the following elements may also be incorporated in the cladding alloy for the improvement of its corrosion-resistance: Pb, Sn, Ni, Cu, Zn, Be, Li and Ge. The role of these additives in the alloy are equated as far as their corrosion-resistance improving effect is concerned.
It has surprisingly been discovered that the presence of lithium in the filler alloy, when added in controlled amounts within the range from about 0.01 to about 0.30% by weight of the alloy, allows the use of the filler alloy for brazing either by the controlled atmosphere brazing method or by the vacuum brazing process.
The universal applicability of the filler alloy of the invention for the brazing of both magnesium-containing and magnesium-free aluminum alloys eliminates the need to segregate these alloys and further provides the freedom to use whichever brazing method is preferred by the manufacturer of brazed aluminum assemblies.